Method of magnetic particle testing using strippable coatings

ABSTRACT

Method for nondestructive magnetic inspection of an object for detecting defects and discontinuities in a surface thereof, by applying a coating, preferably an organic coating, containing a coloring or opaque material, to the object, for contrast therewith, establishing magnetic flux lines at an angle and preferably substantially perpendicular, to the suspected defects and discontinuities in a surface of the object, applying nonfluorescent magnetic particles, preferably suspended in a liquid medium, over the coating on the surface of the object and causing the particles to agglomerate and form indications on the coating adjacent to the surface defects and discontinuities in the object, and inspecting the body under natural or white light to reveal such defects and discontinuities indications as defined by the agglomerated magnetic particles. Where a record of the indications of defects and discontinuities is desired, a strippable colored, e.g. white, coating is initially applied, and following magnetization to locate and reveal the surface defects and discontinuities, a clear strippable coating is applied over the colored coating containing the magnetic indications of defects, and the resulting coating containing the indications formed by the agglomerated magnetic particles can be stripped from the surface of the object, and such coating employed as a record of such indications.

United States Patent [191 Molina Dec. 17, 1974 [75] Inventor: Orlando G.Molina, Westminster,

Calif.

[73] Assignee: Rockwell International Corporation, El Segundo, Calif.

22 Filed: May 16, 1973 211 App]. No.: 360,988

FOREIGN PATENTS OR APPLICATIONS 561,856 6/1944 Great Britain 324/38833,884 5/1960 Great Britain 324/38 OTHER PUBLICATIONS Pevar; M. NewMagnetic Test Includes Stainless Steel; Prod. Eng; Feb. 6, 1961; pp.41-43. McMaster; R. Nondestructive Testing Handbook; Vol. 11, RonaldPress; N.Y. 1963; pp. 30.530.l l.

Primary Examiner-Robert J. Corcoran Attorney, Agent, or Firm-Charles T.Silberberg; L. Lee Humphries [57] ABSTRACT Method for nondestructivemagnetic inspection of an object for detecting defects anddiscontinuities in a surface thereof, by applying a coating, preferablyan organic coating, containing a coloring or opaque material, to theobject, for contrast therewith, establishing magnetic flux lines at anangle and preferably substantially perpendicular, to the suspecteddefects and discontinuities in a surface of the object. applyingnon-fluorescent magnetic particles, preferably suspended in a liquidmedium, over the coating on the surface of the object and causing theparticles to agglomerate and form indications on the coating adjacent tothe surface defects and discontinuities in the object, and inspectingthe body under natural or white light to reveal such defects anddiscontinuities indications as defined by the agglomerated magneticparticles. Where a record of the indications of defects anddiscontinuities is desired, a strippable colored, e.g. white, coating isinitially applied, and following magnetization to locate and reveal thesurface defects and discontinuities, a clear strippable coating isapplied over the colored coating containing the magnetic indications ofdefects, and the resulting coating containing the indications formed bythe agglomerated magnetic particles can be stripped from the surface ofthe object, and such coating employed as a record of such indications 4Claims, 5 Drawing Figures METHOD OF MAGNETIC PARTICLE TESTING USINGSTRIPPABLE COATINGS BACKGROUND OF THE INVENTION This invention relatesto nondestructive testing of bodies by magnetic particle inspectionprocedure, for detection of defects and discontinuities in the surfaceof the body, and is particularly concerned with a method for achievinghigh sensitivity magnetic inspection of bodies by employing relativelyinexpensive nonfluorescent magnetic particles, and to magnetic particlecompositions which can be employed in such method.

Conventional magnetic particle inspection methods for detection ofdefects, flaws and discontinuities in objects or parts, generallyutilize two types of magnetic particles for this purpose, namely,non-fluorescent-type particles usually employed in low sensitivityinspection methods, and fluorescent-type magnetic particles employed inhigh sensitivity inspection methods for detection of minute flaws,defects or microcracks. Also, conventional magnetic particle inspectionmethods generally do not employ a contrasting background against whichthe oriented magnetic particles indicating flaw locations are observed,but generally rely on the contrast between the oriented magneticparticles and the color of the part itself.

In my US Pat. Nos. 3,243,876 and 3,344,345 there is disclosed a magneticinspection method which employes a continuous intermittent alternatingcurrent which is not transmitted through the part, in a printingprocess, and wherein the periodic reversal in the direction of themagnetic force results in violently impinging the magnetic particlesagainst the part surface, leaving a printed mark at the points ofimpingement, following which the magnetic particles are removed from thesurface and inspection is made of the printed pattern resulting fromsuch procedure. According to the patents, the workpiece can be initiallycoated with an opaque material prior to application of the magneticparticles on the object. However, the printing procedure of thesepatents is particularly designed to obtain detailed metallurgicalinformation, for example detailed information as to the honeycombstructure below a facing sheet or below the surface of the body, andrequires specialized equipment for application of a suitableintermittent alternating current to provide the magnetic force to causeviolent impingement of the magnetic particles against the object surfaceto produce the printing smears. In many applications of magneticinspection particle procedures where detailed metallurgical informationsuch as that noted above is not required, and where only surface flawsor discontinuities are required to be detected, it is desirable toemploy less sophisticated and relatively inexpensive conventionalmagnetic inspection equipment and procedure.

Accordingly, a chief object of the present invention is the provision ofa magnetic inspection procedure employing relatively inexpensivenon-fluorescent magnetic particles together with conventional magneticinspection equipment, for detecting surface cracks, flaws anddiscontinuities, with high sensitivity, particularly where minute cracksor microcracks are present.

DESCRIPTION OF THE INVENTION Applicant has surprisingly discovered thatby initially applying a contrasting, e.g., white, background coatingover the surface of the object or part to be inspected, conventionalmagnetic particle inspection techniques can be employed involvingapplication of nonfluorescent-type, e.g., black, magnetic particles andconventional magnetic flux generating apparatus, to obtain contrastingmagnetic particle indications of minute flaws and defects on thecontrasting coating, with a sensitivity range equal to, or even higherthan that obtained utilizing magnetic inspection methods employingexpensive fluorescent magnetic particles and fluorescent or black lightfor illumination.

Thus, according to the present process, a method for nondestructivemagnetic inspection of an object, for detection of defects anddiscontinuities in the surface of the object is provided which comprisesapplying to a surface of the object a coating containing a coloringmaterial, such as an opaque organic coating containing a pigment, e.g.,titanium dioxide, establishing a field of magnetic flux lines relativeto the object so that lines of flux pass at an angle, and preferablysubstantially perpendicular, to the suspected defects in the objectsurface, applying non-fluorescent magnetic particles, preferablysuspended in a suitable liquid medium, over the coating and causing themagnetic particles to agglomerate or group together on the coatingadjacent to or over the surface defects and discontinuities, suchmagnetic particles having a color which contrasts with the color of thecoating, and inspecting the body under natural or white lightillumination to reveal the defects and discontinuities as defined by themagnetic particle indications produced by the agglomerated magneticparticles.

In such process, following application of the colored coating on thesurface of the object, the object or part to be inspected can bemagnetized, according to conventional magnetic inspection procedure, forexample by placing an electromagnet on the coating applied to the partsurface so that when the magnetizing current is turned on, a magneticflux field is generated around the part, with at least some flux linespositioned at an angle, preferably substantially perpendicular, to thediscontinuities or flaws in the surface of the object. While themagnetizing force is on, the magnetic particles are applied, preferablyin the form of a liquid medium such as an organic solvent in which themagnetic particles are suspended, e.g., by spraying, over the coating onthe surface of the object to provide a uniform distribution andconcentration of the magnetic particles over the part surface.

If a record of the flaw indications provided by the pattern of themagnetic particles is required, e.g., to form a permanent record of thelocation and size of the defects and discontinuities in the surface ofthe object, a strippable coating can be initially applied, so thatfollowing magnetization and the agglomeration of the magnetic particlesaccording to the pattern of flaws and discontinuities, the coating canbe stripped from the part and, if desired, copies of the record ofmagnetic indications can be made, e.g., by photographic means.

By provision of the invention process wherein contrasting magneticparticle indications are revealed against a contrasting, e.g., white,background provided by the coating applied to the object, a sensitivityrange equivalent to, or higher than that of the more expensivefluorescent magnetic particle inspection processes can be obtained whileemploying the relatively inexpensive non-fluorescent magnetic particles.Further, the invention procedure only requires the use of ordinary whitelight illumination, and the use of black light or fluorescentillumination ordinarily required for high sensitivity inspectionsemploying fluorescent magnetic particles, is avoided. Also, byemployment of a background coating, contamination and abrasion of thepart by the highly abrasive magnetic particles is avoided. Further, byuse of a background coating, the present process provides a markedly lowlevel of undesirable mechanically entrapped magnetic particles, sincethe coating breaks the natural affinity of the magnetic particles toattach themselves to the surface of the object being inspected, and thuseliminating false indications and also making the part easier to inspectand to clean. Further, as previously noted, use preferably of astrippable coating affords a permanent record of the flaws anddiscontinuities in the part surface.

The detailed practice of the invention process will be understood morereadily from the description below, taken in conjunction with theaccompany drawing wherein FIGS. I to 5 illustrate steps and conditionsin the invention process.

The part or object to be inspected, e.g., in the form ofa plateindicated at in the drawing, and which has magnetic permeability andretentivity, such as a steel or other ferromagnetic metal, usually isfirst cleaned of all foreign material including soil and oily material.Any conventional cleaning procedure can be employed, for example adegreaser such as trichloroethylene.

A colored background coating 12 is then applied to the surface of theobject in sufficient amount to cover the surface to be inspected and tooffset the color of the part being inspected. For this purpose anysuitable coating containing a coloring material can be employed, andsuch coating can be nonstrippable or strippable. Preferably, the coatingis organic and of the type described in my US. Pat. No. 3,279,243. Suchcoatings have an organic polymer base comprised for example of vinyls,acrylics, nitrocellulose, butyrates and latex, the vinyl polymers andcopolymers, such as vinyl chloride resin and vinyl chloride-vinylacetate copolymers, being particularly useful. Such coating compositionincludes a volatile solvent and generally a plasticizer, among suitableplasticizers for this purpose including dioctyl phthalate and butylphthalate. The plasticizer provides suitable flexibility of the resinbinder. A coloring material such as a pigment of a desired color, e.g.white titanium dioxide, is incorporated in suitable amount to color thecoating to produce a contrasting background with respect to the color ofthe part surface. A particularly effective strippable coating asdescribed in Example I of above US. Pat. NO. 3,279,243 is a vinyl basepaint comprising about 19 percent vinyl chloride-vinyl acetate copolymerresin, about 61 percent toluene, 14 percent methyl ethyl ketone, 6percent diisooctyl phthalate and a minor amount of titanium dioxidesufficient to provide the necessary amount of whiteness for contrastwith the color, usually a gray coloration, of the metal object. Astrippable coating composition having substantially the above-mentionedcomposition is marketed as Andrew Brown Brolite White InspectionBackground Coating. However, if desired, non-strippable coatings can beemployed as also described in above US. Pat. No. 3,279,243. The coatingcomposition is applied to the part surface in any convenient manner,preferably by lightly spraying the coating composition on the partsurface, as indicated at 14 in FIG. 1 of the drawing.

The object or part 10 containing the colored or white coating 12 is thenmagnetized employing conventional magnetic particle inspectionequipment. Thus for example, viewing FIG. 2 of the drawing, the legs I6and 18 of an electromagnet 20can be placed over or in contact with thearea of the coating 12 on the part surface, at the location thereonwhere cracks and discontinuities are suspected to be present, and amagnetizing current is turned on. In this manner magnetic flux linesindicated at 22 are generated, passing through the surface of the objectand directed substantially at right angles to the suspected surfaceflaws and discontinuities. For this purpose it is preferred to employ aDC. current, e.g., a half wave rectified DC. current. However, analternating current electromagnet can also be used. Also, if desired,magnetizing apparatus can be employed in which the magnetizing currentpasses through the part being inspected. Alternatively, one or morepermanent, e.g., bar, magnets can be employed to establish the magneticflux field. In order to ensure the location and detection of all cracksand flaws on the part surface, the electromagnet 20 can be raised andthen positioned and oriented at different angles on the coating 12,e.g., to a position at right angles to the position of the electromagnetshown in FIG. 2.

While the magnetizing force is on, the magnetic particles are appliedover the coating 12 on the surface of the object. As previously noted,inexpensive nonfluorescent magnetic particles are employed in thepresent process. The magnetic particles employed preferably have highmagnetic permeability but low retentivity. Coarse magnetic particles canbe employed where cracks are ofa relatively large size which do notrequire high sensitivity. It is preferred to employ fine particle sizemagnetic particles for fine defects or microcracks because of the highsensitivity of such particles. Also a selected combination of varioussize particles can be employed where large cracks or discontinuities aswell as small or minute defects or discontinuities are present in theobject to be tested. Preferably, the magnetic particles are black Fe Oparticles, or for example red magnetic (Fe Q particles can be employed.In either case, where for example a white coating has been initiallyapplied, the black or red magnetic particles provide a high contrastwith the white coating.

Preferably, and particularly where high sensitivity is desired, themagnetic particles are dispersed in a liquid medium. Any suitable liquidmedium can be employed for the magnetic particles. Thus for exampleorganic solvents having a high flash point can be used, suchashydrocarbons, e.g., kerosene. However, water also may be used as acarrier medium for the magnetic particles where no problems arepresented with respect to corrosion of the part. Where organic solventsare employed the excess magnetic particle dispersion can be removed fromthe part by treatment with an organic solvent. As a feature of theinvention, water soluble surfactants such as water soluble ethers ofpolyethylene glycol, e.g., Tergitol nonionic TMN, a trimethyl nonylether of polyethylene glycol containing 6 ethylene oxide groups, orTergitol nonionic NPX, a nonyl phenyl polyethylene glycol ethercontaining 10.5 ethylene oxide groups, can be incorporated into suchorganic solvent media to form novel magnetic particle compositions ordispersions which are removable either by an organic solvent or bywater, as for example to remove excess magnetic particle dispersionapplied to the coating on the surface of the object, either before orafter magnetic particle inspection. Such composition can contain byvolume, about 60 to 90 parts liquid hydrocarbon, e.g., kerosene asvehicle, about 5 to 35 parts water soluble surfactant, and about 2 toparts magnetic particles.

The magnetic particle dispersion or bath is distributed, as by sprayingthe bath stream over the initially applied coating while the magnetizingforce is on, as seen in FIG. 2. If desired, the magnetizing current canbe turned on the instant the bath stream is removed. This insures thatthe greatest concentration of magnetic particles will be distributedover the coating 12 while the current is flowing, so that they can beattracted to any leakage fields created by the defects anddiscontinuities. If the object being inspected is composed of a materialwhich has high magnetic retentivity, the dispersion or bath of magneticparticles alternatively can be applied to the part after the magnetizingforce has been turned off. However, residual magnetic fields aregenerally weaker than magnetic fields when magnetizing current isflowing, and hence inspection by use of residual fields is lesssensitive.

Although the magnetic particles can be applied directly as by dustingonto the coating 12, to be magnetized thereon as noted above, thisprocedure is not preferred if high sensitivity is to be obtained.

As result of the application of the magnetic field so that the fluxlines 22 preferably are substantially perpendicular to the surfacecracks or discontinuities, e.g., as indicated at 24 in FIG. 3, andexaggerated therein for greater clarity, due to variations in magneticpermeability of the part at the location of the cracks anddiscontinuities, in effect minute north N and south S poles are createdon opposite sides of such cracks and discontinuities. This causes themagnetic particles 26 to group together or to agglomerate, on thecoating 12 across the N and S poles and over the coating which may coverthe mouth of the cracks 24, to form a magnetic particle indication 28 ofsuch cracks or discontinuities, on the contrasting colored coating.

It is particularly significant that particularly in the case of minutecracks and discontinuities, where the coating is continuous over thecrack, as indicated at 12' in FIG. 3, the magnetic particles agglomerateand group together as at 28 to form the particle indications, thusincreasing the sensitivity of the inspection process. However, in thecase of relatively large or coarse cracks, as indicated at 24 in FIG. 4,where the coating is discontinuous at the mouth of the crack, asindicated at 29, the magnetic particles will agglomerate across themouth of the open crack and supported on the coating at opposite sidesof the crack, and thus provide magnetic indications 28 also of suchcoarse cracks or discontinuities.

The object or part is then inspected as illustrated in FIG. 5 undernatural or white light illumination indicated at 30 for visualindications of cracks, flaws and discontinuities. The resultingindications 28 of the cracks and discontinuities thus produced by suchagglomeration of the magnetic particles are highly visible when viewedunder natural or white light illumination, and appear as sharp blacklines where black magnetic particles are employed, or sharp red lineswhere for example red magnetic particles are employed, against theintense, e.g. white, background of the coating.

lf following inspection of the part, a record of the indications is notrequired, the coating containing the magnetic particle indicationsthereon can be removed by treatment in a suitable solvent or by waterwashing depending upon whether the coating initially applied is anorganic solvent soluble or water washable system.

However, if a record of the magnetic indications is required, and astrippable colored coating has been employed initially, the object orpart following visual inspection first can be gently bathed in a solventsuch as naphtha, trichloroethane or a Freon, i.e., a fluorocarbon suchas Freon TF, which is 1,l,2-trichloro-l,2,2- trifluoroethane, where anorganic medium e.g. kerosene-type, dispersion of the magnetic particleswas employed, to remove any remaining magnetic particle dispersion ororganic medium thereof. With this gentle treatment, the magneticindications 28 will not be washed off since the magnetic attractionprovided by the magnetic retentivity of the object will hold theagglomerated magnetic particles forming the indications together overthe location of the cracks and discontinuities. After the above-notedtreating solvent, e.g., naphtha, dries, the indications can be frozen"into the coating by spryaing the coating with a clear quickdrying resinsolution such as a clear strippable vinyl lacquer, which blends with oradheres to the initial colored, e.g., white, coating, and forms astrippable coat which serves to protect the magnetic indications frombeing dislocated on the initial colored coating. lt should be noted thatboth the initial colored background, e.g., white vinyl, coating and theclear, e.g., vinyl, coating are strippable, but in effect both coatingsform a single strippable coating containing the magnetic indications.

After drying of the clear, e.g., vinyl, overcoat, the entire coatingsystem including the initially applied contrasting strippable coatingcontaining the frozen" magnetic indications, and the clear strippable,e.g., vinyl, overcoat, can be stripped by lifting from an edge thereof.If the clear resin coating is too thin, a reinforcing application of aclear plastic tape such as a vinyl tape, can be applied over the clearresin overcoat. Examples of such reinforcing plastic tape are thosemarketed as 3M Clear Magic Tape or 3M Clear Plastic Tape No. 471. Theresulting reinforced coating can be readily detached or stripped fromthe surface of the part. Copies of the resulting recording of themagnetic indications of the defects and discontinuities in the part canbe obtained by using the stripping as a negative, e.g., for contactprinting.

The following are examples of practice of the invention.

EXAMPLE 1 A steel test specimen or part containing extremely smallcracks or microcracks was first cleaned of soil and oily materials bytreatment with a trichloroethylene degreaser.

The surface of the steel specimen was then coated with a coatingcomposition in the form of a vinyl base paint comprising about 19percent vinyl chloride-vinyl acetate copolymer resin, 1 percent whitemineral oil, about 61 percent toluene, 14 percent methyl ethyl ketoneand 6 percent diisooctyl phthalate, and a small amount of non-chalkingtitanium dioxide (rutile) sufficient to provide the required amount ofwhiteness for contrast with the grayish color of the part surface. Thecoating composition contained sufficient methyl ethyl ketone as thinnerto permit spraying of the coating composition on the part surface. Aspreviously noted, such coating composition is a strippable coating.

The part was sprayed with the white coating in sufficient amount tooffset the color of the steel specimen, and the coating was permitted todry for a period of about 1 to 2 minutes.

The coated part was then magnetized by placing the conducting legs of anelectromagnet of a conventional magnetic particle inspecion apparatus ofthe type illustrated in FIG. 2, on the coating and employing a half waverectified D.C. magnetizing current. By suitable orientation of the legsof the electromagnet on the coating, the magnetizing current generateslines of flux in the part surface in a direction approximatelyperpendicular to the defects, cracks and discontinuities to be detectedtherein.

With the magnetizing current on, a suspension or dispersion of very fineblack magnetic particles Fe,o,) was distributed, as by spraying, overthe white coating on the surface of the object and between the heads ofthe electromagnet. The magnetic particle suspension had the followingcomposition:

(black Fe OQ Excess magnetic particle dispersion was removed by gentlewater spraying.

Following application of the suspension over the coating and orientationof the magnetic particles by the magnetic flux to form agglomeratedparticle indications on the coated surface of the object, themagnetizing current was turned off and the part was inspected undernatural or white light illumination for indications of the cracks anddiscontinuities in the part surface. The magnetic particle indicationsof the fine cracks or microcracks in the steel part were highly visibleand appeared as sharp black lines against the intense white backgroundcoating.

As this stage, the part could be repaired by grinding through thecoating to remove the cracks for welding.

EXAMPLE 2 A record of the magnetic indications of cracks anddiscontinuities in the part treated according to Example l was obtained.For this purpose the steel object or part containing the white coatingand black magnetic particle indications produced by the agglomeratedmagnetic particles over the discontinuities of the part, was firstbathed gently in naphtha solvent to remove any remaining magneticparticle dispersion or kerosene solvent from the magnetic particledispersion, but leaving the magnetic indications intact at their initiallocations on the coating. After the naphtha solvent dried, the magneticindications were "frozen in place by spraying a coating of clearstrippable vinyl lacquer over the initial white strippable vinyl coatingand magnetic particle indications.

After the vinyl overcoating dried, the entire coating system wasstripped as a unit from the part by lifting from one edge, and theresulting stripping was used as a negative for contact printing toobtain copies of the recorded magnetic indications.

EXAMPLE 3 The procedure of Example 1 was repeated but wherein themagnetic particle suspension employed was changed to omit the TergitolNonionic TMN component and adjusting the kerosene content to percent byvolume.

Following inspection of the part by means of natural white light, thewhite coating and the magnetic particle indications on the coating wereremoved by treatment with methyl ethyl ketone solvent.

EXAMPLE 4 The procedure of Example 1 was repeated but employing in placeof the strippable coating employed in Example l, a compositioncomprising a mixture of ethyl cellulose and titanium dioxide in the formof a commerical product marketed as Logo, to which a solvent vehicle ofdenatured alcohol was added. Following inspection under natural or whitelight to obtain magnetic particle indications of the minute flaws anddiscontinuities of the part, the white contrasting coating and themagnetic particle indications thereon were removed from the part bywater washing.

EXAMPLE 5 The procedure of Example 1 was repeated except employing adifferent type of magnetic particle inspection apparatus of aconventional type wherein the part was placed between the conductingheads of such apparatus and a DC. magnetizing current was employed,which passes through the part. Magnetic indications were obtained of asensitivity comparable to the results of Example 1.

From the foregoing, it is seen that the invention provides a method forobtaining high sensitivity magnetic particle inspection of an object orpart, by employment of relatively inexpensive non-fluorescent magneticparticles which are generally employed in low sensitivity magnetic.particle inspection methods, in conjunction with an initial coatingcontaining a coloring material, preferably in the nature of a pigmentsuch as titanium dioxide, providing high contrast between the partsurface and the coating, and between the coating and the magneticparticle indications, and permitting high sensitivity inspection, thusavoiding the use of relatively expensive fluorescent magnetic particlesand black light illumination, usually required for high sensitivityinspection in prior art processes. The invention also provides novelmagnetic particle dispersion compositions for use in the inventionprocess.

While I have described particular embodiments of my invention for thepurpose of illustration within the spirit of the invention, it will beunderstood that the invention is not to be taken as limited except bythe scope of the appended claims.

I claim:

1. A method for nondestructive magnetic inspection of an object, fordetection of defects and discontinuities in the surface of said object,which comprises applying a strippable coating containing a coloringmaterial to said object, said colored coating contrasting with the colorof said object, establishing a field of magnetic flux lines relative tosaid object so that lines of flux pass at an angle to said defects anddiscontinuities in the surface of said object, applying a suspension ofnonfluorescent magnetic particles in a liquid medium over said coloredcoating, said suspension consisting essentially of a liquid hydrocarbonvehicle, a water soluble surfactant and said magnetic particles, andcausing said magnetic particles to agglomerate on said coating adjacentto said defects and discontinuities, said magnetic particles having acolor which contrasts with the color of said coating, to providemagnetic particle indications of said defects and discontinuities,gently treating said coated surface containing said magnetic particleindications in a solvent, said magnetic particle indications beingmaintained in their initial agglomerated positions, applying over saidstrippable coating and over said magnetic particle indications, a secondclear, strippable coating, and stripping the coating system includingsaid colored coating containing said magnetic particle indications andsaid clear coating from the surface of said object, to provide a recordof said defects and discontinuities in the surface of said object.

2. A method for nondestructive magnetic inspection of an object, fordetection of defects and discontinuities on the surface of said object,which comprises spraying a vinyl base paint containing titanium dioxideon a magnetically permeable object, drying said coating, said driedcoating being strippable, establishing a field of magnetic flux linesrelative to said object by employing a substantially non-fluctuatingD.C. magnetizing current so that lines of flux pass through the surfaceof said object substantially perpendicular to the defects anddiscontinuities therein, distributing over said coated surface asuspension consisting essentially of black magnetic particles in akerosene medium and a water soluble surfactant to render said suspensionwater washable, said field of flux lines causing said particles toagglomerate on said coating adjacent said defects and discontinuities insaid part surface, to provide magnetic particle indications of saiddefects and discontinuities, gently treating said coated surfacecontaining said magnetic particle indications in a solvent, saidmagnetic particle indications being maintained in their initialagglomerated positions, applying over said strippable coating and oversaid magnetic particle indications, a second clear, strippable coating,and stripping the coating system including said colored coatingcontaining said magnetic particle indications and said clear coatingfrom the surface of said object, to provide a record of said defects anddiscontinuities in the surface of said object.

3. A method as defined in claim 2, wherein said solvent is selected fromthe group consisting of naphtha, trichloroethane and a fluorocarbon,wherein said second clear coating is applied by spraying a coating of aclear strippable vinyl over said vinyl coating containing titaniumdioxide, and over said magnetic particle indications, and drying saidlast mentioned coating.

4. A method as defined in claim 3, including applying over said clearvinyl strippable resin coating a clear vinyl plastic tape to reinforcesaid clear vinyl coating, prior to stripping of the coating system.

1. A METHOD FOR NONDESTRUCTIVE MAGNETIC INSPECTION OF AN OBJECT, FORDETECTION OF DEFECTS AND DISCONTINUITIES IN THE SURFACE OF SAID OBJECT,WHICH COMPRISES APPLYING A STRIPPABLE COATING CONTAINING A COLORINGMATERIAL TO SAID OBJECT, SAID COLORED COATING CONTRASTING WITH THE COLOROF SAID OBJECT, ESTABLISHING A FIELD OF MAGNETIC FLUX LINES RELATIVE TOSAID OBJECT SO THAT LINES OF FLUX PASS AT AN ANGLE TO SAID DEFECTS ANDDISCON TINUITIES IN THE SURFACE OF SAID OBJECT, APPLYING A SUSPENSION OFNON-FLUORESCENT MAGNETIC PARTICLES IN A LIQUID MEDIUM OVER SAID COLOREDCOATING, SAID SUSPENSION CONSISTING ESSENTIALLY OF A LIQUID HYDROCARBONVEHICLE, A WATER SOLUBLE SURFACTANT AND SAID MAGNETIC PARTICLES, ANDCAUSING SAID MAGNETIC PARTICLES TO AGGLOMERATE ON SAID COATING ADJACENTTO SAID DEFECTS AND DISCONTINUITES, SAID MAGNETIC PARTICLES HAVING ACOLOR WHICH CONTRASTS WITH THE COLOR OF SAID COATING, TO PROVIDEMAGNETIC PARTICLE INDICATONS OF SAID DEFECTS AND DISCONTINUITES, GENTLYTREATING SAID COATED SURFACE CONTAINING SAID MAGNETIC PARTICLEINDICATIONS IN A SOLVENT, SAID MAGNETIC PARTICLE INDICATIONS BEINGMAINTAINED IN THEIR INITIAL AGGLOMERATED POSITIONS, APPLYING OVER SAIDSTRIPPABLE COATING AND OVER SAID MAGNETIC PARTICLE INDICATIONS, A SECONDCLEAR, STRIPPABLE COATING, AND STRIPPING THE COATING SYSTEM INCLUDINGSAID COLORED COATING CONTAINING SAID MAGNETIC PARTICLE INDICATIONS ANDSAID CLEAR COATING FROM THE SURFACE OF SAID OBJECT, TO PROVIDE A RECORDOF SAID DEFECTS AND DISCONTINUITIES IN THE SURFACE OF SAID OBJECT.
 2. Amethod for nondestructive magnetic inspection of an object, fordetection of defects and discontinuities on the surface of said object,which comprises spraying a vinyl base paint containing titanium dioxideon a magnetically permeable object, drying said coating, said driedcoating being strippable, establishing a field of magnetic flux linesrelative to said object by employing a substantially non-fluctuatingD.C. magnetizing current so that lines of flux pass through the surfaceof said object substantially perpendicular to the defects anddiscontinuities therein, distributing over said coated surface asuspension consisting essentially of black magnetic particles in akerosene medium and a water soluble surfactant to render said suspensionwater washable, said field of flux lines causing said particles toagglomerate on said coating adjacent said defects and discontinuities insaid part surface, to provide magnetic particle indications of saiddefects and discontinuities, gently treating said coated surfacecontaining said magnetic particle indications in a solvent, saidmagnetic particle indications being maintained in their initialagglomerated positions, applying over said strippable coating and oversaid magnetic particle indications, a second clear, strippable coating,and stripping the coating system including said colored coatingcontaining said magnetic particle indications and said clear coatingfrom the surface of said object, to provide a record of said defects anddiscontinuities in the surface of said object.
 3. A method as defined inclaim 2, wherein said solvent is selected from the group consisting ofnaphtha, trichloroethane and a fluorocarbon, wherein said second clearcoating is applied by spraying a coating of a clear strippable vinylover said vinyl coating containing titanium dioxide, and over saidmagnetic particle indications, and drying said last mentioned coating.4. A method as defined in claim 3, including applying over said clearvinyl strippable resin coating a clear vinyl plastic tape to reinforcesaid clear vinyl coating, prior to stripping of the coating system.